Oncogenes have become the central concept in understanding cancer biology and may provide valuable targets for therapeutic drugs. All oncogenes and their products operate inside the cell. This makes protein-based drugs ineffective since their specificity involves ligand-receptor recognition.
Antisense oligodeoxyribonucleotides (oligonucleotides) are under investigation of therapeutic compound for specifically targeting oncogenes (Wickstrom, E. (ed). Prospects for antisense nucleic acid therapy of cancer and Aids. New York: Wiley-Liss, Inc. 1991; Murray, J. A. H. (ed). Antisense RNA and DNA New York: Wiley-Liss, Inc. 1992). Antisense drugs are modified synthetic oligonucleotides that work by interfering with ribosomal translation of the target mRNA. The antisense drugs developed thus far destroy the targeted mRNA by binding to it and triggering ribonuclease H (RNase H) degradation of mRNA. Oligonucleotides have a half-life of about 20 minutes and they are therefore rapidly degraded in most cells (Fisher, T. L. et al., Nucleic Acids Res. 21:3857-3865 (1993)). To increase the stability of oligonucleotides, they are often chemically modified, e.g., they are protected by a sulfur replacing one of the phosphate oxygens in the backbone (phosphorothioate) (Milligan, J. F. et al., J. Med. Chem. 36:1923-1937 (1993); Wagner, R. W. et al., Science 260:1510-1513 (1993)). However, this modification can only slow the degradation of antisense and therefore large dosages of antisense drug are required to be effective.
Despite the optimism surrounding the use of antisense therapies, there are a number of serious problems with the use of antisense drugs such as difficulty in getting a sufficient amount of antisense into the cell, non-sequence-specific effects, toxicity due to the large amount of sulfur containing phosphothioates oligonucleotides and their inability to get into their target cells, and high cost due to continuous delivery of large doses. An additional problem with antisense drugs has been their nonspecific activities.
What is needed are additional non-protein based cancer therapeutics that target oncogenes. Therapeutics that are effective in low doses and that are non-toxic to the subject are particularly needed.